cmd: CONFIG_CMD_SAVES depends on CONFIG_CMD_LOADS

CONFIG_CMD_SAVES is used to enable support for the "saveenv" command
and is only implemented in cmd/load.c
when "#if defined(CONFIG_CMD_LOADS)" is met.

It is recommended to add dependency constraints to its definition.
Prevents "saveenv" command from not being supported
when "CONFIG_CMD_SAVES=y CONFIG_CMD_LOADS=n".

Suggested-by: Yanjie Ren <renyanjie01@gmail.com>
Signed-off-by: Ying Sun <sunying@nj.iscas.ac.cn>
Reviewed-by: Simon Glass <sjg@chromium.org>